1. Field of the Invention
The invention relates to a device and a method for detecting an abnormality of variation of air-fuel ratio among cylinders. In particular, the invention relates to a device and a method that detect that the air-fuel ratio is relatively greatly varied among cylinders in a multi-cylinder internal combustion engine.
2. Description of Related Art
In general, in an internal combustion engine that is equipped with an exhaust gas purification system that utilizes a catalyst, noxious components in exhaust gas are highly efficiently purified by the catalyst. Thus, the control of the mixing ratio between air and fuel in the air-fuel mixture burned in the internal combustion engine, namely, the air-fuel ratio is indispensable. In order to control this air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and feedback control is performed such that an air-fuel ratio detected by this air-fuel ratio sensor coincides with a predetermined target air-fuel ratio.
On the other hand, in the multi-cylinder internal combustion engine, the same controlled variable is usually used for all the cylinders to perform air-fuel ratio control. Thus, even when air-fuel ratio control is performed, the actual air-fuel ratio may vary among the cylinders. In this case, if the degree of variation is small, the variation can be absorbed through air-fuel ratio feedback control, and noxious components in exhaust gas can be treated to be purified by the catalyst as well. Therefore, the exhaust emission properties are not influenced, and no problem is caused in particular.
However, for example, if the air-fuel ratio greatly varies among the cylinders due to a malfunction in a fuel injection system or fuel injection systems in one or some of the cylinders or the like, the exhaust emission properties deteriorate to such a degree as to cause a problem. Such a great air-fuel ratio variation as to cause a deterioration in the exhaust emission properties is desired to be detected as an abnormality. In particular, in the case of an internal combustion engine for a motor vehicle, there have been demands to detect an abnormality of variation of air-fuel ratio among the cylinders in an in-vehicle state (onboard) in order to prevent vehicles whose exhaust emission properties have deteriorated from traveling. Recently, there have also been moves to enshrine this detection into law.
For example, in a device described in Japanese Patent Application Publication No. 2009-180171 (JP-2009-180171 A), an abnormality of variation of air-fuel ratio among the cylinders is detected on the basis of fluctuations in the air-fuel ratio of the internal combustion engine. Furthermore, as for a plurality of fuel injection valves provided in a plurality of the cylinders respectively, the injection ratio among the plurality of these fuel injection valves is changed among a plurality of predetermined ratios. It is then identified, on the basis of fluctuations in the air-fuel ratio before and after this change, which one of the fuel injection valves constitutes a cause of the variation abnormality.
However, in the configuration of Japanese Patent Application Publication No. 2009-180171 (JP-2009-180171 A), if the injection amount of any one of the fuel injection valves is small when the injection ratio is changed, the accuracy of the control of the injection amount becomes low. Therefore, a targeted air-fuel ratio cannot be realized. For this reason, it may become difficult to identify which one of the fuel injection valves is abnormal.